Historically, gas turbine engines have used a wide variety of hydrocarbon fuels such as natural gas, jet fuel, and diesel fuel. For these fuels, flame temperatures in air can exceed 3000° F. and reaction rates thus become sufficiently fast that complete combustion is easily achieved. For low-Btu fuels, however, such as gasified coal, blast furnace gas, or landfill or other waste gases, sufficient diluent may be present that flame temperatures and reaction rates are reduced to the point that combustion is difficult to sustain. Thus, a catalyst may be employed to increase the rate of reaction until gas-phase combustion can be sustained.
One attractive option for catalytic combustion of low-Btu fuels is to use the method and apparatus described in U.S. Pat. No. 6,358,040 and its divisional U.S. Pat. No. 6,394,791 (respectively, “the '040 Patent ” and “the '791 Patent”). These patents describe an air-cooled catalytic reactor comprising metal conduits or tubes having catalyst-coated exterior surfaces. In operation, fuel is mixed with air in fuel-rich proportions and contacted with the catalyst. A separate air stream passes through the tubes' interiors to cool the catalyst. At the reactor exit (the downstream end of the tubes), the cooling air stream mixes with the catalytically-reacted fuel-rich stream to create a fuel-lean mixture to promote complete combustion.
In the embodiment depicted in the '791 patent, the reactor comprises round tubes passing through a reactor housing, where such tubes are held at their upstream ends by attachment to a perforated plate. At their downstream ends, however, it is preferred that the tubes are self-supported by simply resting against one another. This self-supporting arrangement avoids the need for an additional support structure in the reactant stream outside the tubes, where the possibility of gas-phase reactions and high temperatures complicates the design requirements of such a structure. In contrast, the tubes themselves are already air-cooled and designed to withstand a high-temperature environment.
As also described in the '791 patent, it is preferred that the tubes are spaced apart along most of their length within the reactor housing, thus creating a single catalytic reaction channel around the exterior surface of the tubes and the interior of the housing thereby allowing the reactant stream to enter the reaction channel from the side through an aperture in the housing, crossing over the tubes. This provides complete separation between the reactant stream and the cooling air stream which enters the tube interiors through the perforated plate. Thus, it is preferred that the tubes are expanded (flared) over a portion of their length, at or near their downstream ends (opposite the plate end), so that adjacent tube flares touch each other but the tubes are otherwise separated. This is most easily accomplished by expanding round tubes to a larger round diameter at their downstream ends, as shown in FIG. 9 of the '791 patent.
Thus, in a practical, easy-to-manufacture implementation of the '791 patent's catalytic reactor, the downstream end of the reactor becomes an arrangement of contacting circles having equal diameters. Various packing arrangements are possible. For example, the circles may be placed in a square-pack arrangement, as shown in FIG. 1 herein. In this packing arrangement the space between the contacting circles, from which all catalytically reacted effluent must exit, comprises more than 20%, a desirable feature allowing lower pressure drop designs. However, the square-pack arrangement has a disadvantage: for reactors having many tubes, if one tube moves out of position then other tubes may shift position, and in fact the entire assembly of tubes tends to shift position. There are many reasons a single tube might move out of position. The single tube might have a slightly smaller diameter than the others; it may be out-of-round; or, if the tube contacts the housing of the reactor, it may shift position if the housing is not dimensioned correctly. Because the fit of one tube with adjacent tubes affects the position of other tubes, practitioners characterize the square-pack arrangement as “unstable.” For the purpose of this description, an “unstable” packing is one wherein multiple tubes can shift position if one tube moves.
A “stable” packing, on the other hand, results when the tubes are placed in a close-pack arrangement, as shown in FIG. 2 the tightest arrangement possible. Here, each tube contacts six adjacent tubes. Even if a single tube is removed, the remaining tubes will stay in their original positions. This is evident in FIG. 2: if tube A is removed, tube B will stay in position because it is held in place by tubes C and D, and by tubes E. Likewise, tube C will stay in place, as will tube D and tubes E. Tubes C and D prevent tube B from moving because the distance between the contact point where tubes B and C touch, and the contact point where tubes B and D touch, is less than the flared diameter of tube B (i.e. the diameter of tube B shown in FIG. 2, in the plane where it contacts tubes C, D, and E). Thus tube B is held in place, wedge-like, between tubes C and D.
In general, lateral (radial) support is best provided when the tubes are packed in their densest (close-packed) configuration as shown in FIG. 2. However, in this packing arrangement the space between the contacting circles, from which all catalytically reacted effluent must exit, comprises less than about 10% of the total gas flow area, depending on tube wall thickness, the remaining flow area being supplied by the cooling air exits inside the tube circles and the sum of the tube wall areas. For catalytic combustion of hydrocarbon fuels such as natural gas, it is preferred that approximately 15% or more of the total air pass through the catalytic reaction channel, with the remainder passing through the inside of the tubes as cooling fluid. Thus, a fuel-rich mixture having an equivalence ratio (the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio; an equivalence ratio greater than one defines a fuel-rich fuel/air mixture, and an equivalence ratio less than one defines a fuel-lean fuel/air mixture) of about 3 reacts on the catalyst, and then mixes with the cooling air upon exiting the reactor to give an overall equivalence ratio of about 0.5. For this example, when the natural gas fuel is added, approximately 20% of the total volume of fluid (fuel plus air) passes through the catalytic reaction channel and must exit from the small area between the contacting circles at the reactor's downstream end, causing undue pressure drop. Accordingly, reactor designs providing catalytic channel exit areas greater than that of close pack reactors by at least twenty percent are needed. In contrast, by positioning a restrictor plate upstream of the cooling holes, one may create extra undesirable pressure drop and increase the split.
For other fuels, even higher catalytic channel exit areas are often needed. For example, for low-Btu fuels such as coal-derived syngas, blast furnace gas, or waste gas from industrial or biological processes, the fuel's heating value per unit volume of fuel gas can be 25% (or less) of the natural gas heating value, requiring greatly increased volume flow of fuel. Thus if a syngas fuel, having 25% of the heating value of natural gas, is mixed with air to provide an equivalence ratio of about 3 and then reacted on the catalyst, more than 30% of the total volume of fluid passes through the catalytic reaction channel is fuel gas(assuming an overall equivalence ratio of about 0.5 after mixing with catalyst cooling air downstream of the reactor). Again, this 30% of total volume must pass through the total cross-sectional area at the reactor's downstream end, causing further undue pressure drop. Frequently, heating values are well below 25% of the natural gas value, requiring even greater flow of fluid through the catalytic reaction channel.
Based on the foregoing, it is an objective of the present invention to provide a method and apparatus for an improved configuration of the elements comprising a catalytic reactor having a catalytic reaction channel and separate cooling air stream channels. It is a further objective of the present invention to retain the benefits of manufacturing ease and positive lateral support via a stable packing arrangement within the catalytic reactor. It is yet another objective of the present invention to provide greater flow area between the contacting tube element exits.